In batteries, ions transfer between the negative electrode and positive electrode during charge and discharge cycles. For instance, when discharging, electrons flow from the negative electrode, through an external circuit, to the positive electrode to generate an electrical current in the external circuit. During this process, positive ions, for example lithium ions in a lithium-ion battery, travel within the battery from the negative electrode, through an electrolyte, to the positive electrode. Conversely, when charging, the external circuit supplies current that reverses the flow of electrons from the positive electrode, through the external charging circuit, and back to the negative electrode, while the positive ions move within the battery from the positive electrode through the electrolyte to the negative electrode.
Two important measures by which the performance of batteries are determined are the energy density of the battery, or the ratio of the energy stored to the volume or size of the battery, and the rate at which the battery can be charged or discharged. In conventional batteries, there is a tradeoff between the energy density of the battery, and the rate at which the battery can be charged or discharged. For a given set of battery materials, the energy and charge/discharge rate can be modified by, for example, changing the quantity of active material in the electrodes. The amount of active material in the electrodes can be increased by either decreasing the pore space occupied by the electrolyte, or increasing the thickness of the electrode. Either of these modifications, however, leads to a decrease in the rate at which the cell can be charged or discharged.
One particular limiting factor to the charge and discharge speed of a battery is known as concentration polarization. As noted above, during charging and discharging, the ions in the electrolyte adjacent to the positive electrode and negative electrode, respectively, travel from the electrodes through the electrolyte and to the other of the negative electrode and positive electrode, respectively. The counter-ions, or the ions that do not participate in the reactions at the negative electrode and positive electrode, tend to migrate in the direction opposite of the direction the active ions are moving. During charging, the ion travel can result in ion accumulation at the positive electrode and ion depletion at the negative electrode. Conversely, during discharge, the ion travel results in ion accumulation at the negative electrode and ion depletion at the positive electrode. This ion accumulation and depletion is known as concentration polarization.
The concentration polarization in the electrolyte immediately adjacent to the positive electrode or negative electrode reduces the speed at which the battery can be charged and discharged since there are a reduced number of active ions that can participate in the electrochemical reactions adjacent to the respective electrode. Furthermore, in some instances the active ions adjacent to the positive electrode and/or negative electrode can be depleted to the extent that undesirable reactions occur in the battery, causing damage to the battery.
Some conventional batteries attempt to reduce concentration polarization by increasing the mobility of the reactive ions in the battery. However, increasing the mobility of the reactive ions requires redesign of the electrolyte in the battery, which can involve a host of further considerations, can increase the cost of the battery, and can reduce the efficiency of the battery in other ways.
What is needed therefore is an alternative way of reducing the concentration polarization of a battery to improve the efficiency and performance of the battery.